In order to provide more accurate diagnosis by means of medical images, the magnetic resonance imaging (MRI) technique is widely applied in the clinical diagnosis because it could provide the higher resolution and is unlike X-ray that causes the radiation damage. MRI is a phenomenon of the image contrast of the different relaxivities under the strong magnetic field, and the degree of the image contrast could be enhanced in the presence of the contrast agents.
According to the properties of the magnetic field, the contrast agents could be classified into two types. One is T1 contrast media that is designed to reduce the spin-lattice relaxation time (T1). This kind of T1 contrast agents enhances the strength of T1 signals, and the target tissue signal is thus strengthened (looks brighter in the MRI image). Up to the present time, there are several commercial T1 contrast agents that belong to T1 metal chelates, such as a [Gd(DTPA)]2− (diethylenetriaminepentaacetate-gadolinium(III)), a [Gd(DOTA)]− (1,4,7,10-tetraazacyclododecane-N,N′,N″,N″′-tetraacetate-gadolinium (III)), a [Gd(BOPTA)]2− (benzyloxypropioic-diethylenetriamine pentaacetate-gadolinium(III)) and a MnDPDP(N,N′-dipyridoxylethylene diamine-N,N′-diacetate-5,5′-bis(phosphate)-manganese(II)).
The other one is T2 contrast media that is designed to reduce the spin-spin relaxation time (T2). This kind of T2 contrast media is a polymer of particulates that is usually named as superparamagnetic iron oxide (SPIO) nanoparticles and could cause the phenomena of the field inhomogenicity that further shortens the T2 relaxation time. Therefore, T2 contrast media can be used to reduce the target tissue signals so as to identify whether the tissue is normal (looks darker in the MRI image).
The curative effect of the T1 contrast agents is limited to the area within blood vessels, and thus its application is rather narrower. Nevertheless, the application of the T2 contrast agents can be broadened to the peripheral area of the blood capillary. Therefore, the T2 contrast media having the SPIO particles is of the great applicable value, wherein the target-oriented magnetic contrast agents become an important subject for the researches in the magnetic image field.
However, the iron oxide nanoparticles easily tend to aggregate and absorb plasma proteins due to their higher surface areas. While these iron oxide nanoparticles are injected into human bodies, they will rapidly be eliminated by the mononuclear phagocytes system (MPS), removed from the blood and thus fail to arrive at the target tissue cells if the aggregation or absorption of the human macrophages is occurred. Therefore, in order to increase the half-life of those iron oxide nanoparticles circulating in the blood, a specific thin layer is needed to cover the surfaces thereof. Furthermore, the material of this thin layer should be of the higher biological compatibility, nonimmunogenity, nonantigenity and protein-resistance. After covered with the specific thin layer, the aggregation and the absorption of macrophages in the human body will be highly prevented, which reduces the numbers of those iron oxide nanoparticles to be engulfed by the mononuclear macrophage. Accordingly, those iron oxide nanoparticles could successfully arrive at the target tissue cells.
The current thin layer for covering the SPIO particles is commonly composed of dextrans or its derivatives. Poly(ethylene glycol) (PEG) is a hydrophilic residue of no charges and with low toxicity, which can be metabolized in the human body. In the recent decade, the biological compatibilities of many drugs are improved by attaching PEG compounds thereto, which might prevent the absorption of the proteins. Hence, a huge amount of PEG compounds is necessary for the modification of the surface of those iron oxide nanoparticles. However, the preparation of the current commercial PEG compounds, such as those comprising —NHS or —COOH groups in the market, should expend multiple complicated and synthetic steps to be more reactive. Correspondingly, the commercial prizes of these PEG compounds with modified functional groups are highly expensive.
There are several ways to internalize the iron oxide nanoparticles, such as the fluid phase endocytosis, the receptor-mediated endocytosis and the phagocytosis. Through the mechanism of the receptor-mediated endocytosis, the iron oxide nanoparticles can be designed to be cell-specific by providing a probe on the surface thereof that target to the specific cell membranes. Despite the magnetic contrast media of iron oxide nanoparticles have the great potential for different applications, there are only Resovist®, Feridex®, Endorem™ , GastroMARK® and Lumirem® that has acquired the FDA permission to practically apply in the image diagnosis. Furthermore, the mentioned contrast agents are all designed to target to the normal cells. If the probe on the surface of SPIO particles could be designed to target to the damaged cells, the application of SPIO particles could be further broadened.
Folic acid (FA) is a soluble vitamin B and also a key precursor for the synthesis of DNA and RNA. The normal cell membranes have a specific amount of receptors for coupling to the FA; the tumor cell membranes, however, will over-express the FA receptors whose amount is several times than those of the normal cell membranes. Therefore, if the SPIO particles are designed to target to the FA receptors, they could be applied in the diagnosis for positioning the tumor cells and the further treatment.
In J. Am. Chem. Soc. Vol. 126, P7206 (2004), M. Zhang et al. disclosed that the PEG compounds with the average molecular weight of 600 Daltons was provided to cover the surface of iron oxide nanoparticles. Through the identification of the FT-IR spectrometer, it was proved that the surface of the iron oxide nanoparticles is modified with PEG and FA. But the synthesis of the PEG compounds that are used to modify iron oxide nanoparticles still takes the complicated and the expensive procedures as well as there needs a kind of contrast agents capable of targeting the specific tissue cell membranes in the medical-image diagnosis.
In order to overcome the drawbacks in the prior art, a SPIO-PEG-FA compound capable of concisely identifying the position of those tumor cells through the magnetic resonance image is provided. The particular design in the present invention not only solves the problems described above, but also is easy to be implemented. Thus, the invention has the utility for the industry.